Muscle & peripheral nerve nontumor
Inflammatory myopathies

Topic Completed: 1 June 2016

Minor changes: 12 January 2021

Copyright: 2002-2021,, Inc.

PubMed Search: Muscle [title] + Polymyositis [title]

Meggen Walsh, D.O., M.S., P.A.
Page views in 2020: 3,831
Page views in 2021 to date: 1,688
Cite this page: Walsh M. Polymyositis. website. Accessed May 15th, 2021.
Definition / general
  • Inflammatory myopathy that targets predominantly skeletal muscle
  • Can be seen in other autoimmune diseases
Essential features
  • Skeletal muscle shows myopathic changes with evidence of myofiber degeneration and regeneration with lymphocytes invading non necrotic myofibers
  • Predominant inflammatory cell is the CD8+ T lymphocyte
  • Polymyositis or idiopathic polymyositis
  • Prevalence worldwide is 5 - 21.5 per 100,000 (Ann Rheum Dis 2009;68:1192), but only 3.8 per 100,000 in US (Muscle Nerve 2012;45:676)
  • Peak incidence is in older patients, with a mean age of 56 years and a male to female ratio of 1.6 : 1
  • Extremely rare in younger patients and childhood
  • Not fully known
  • In a large study from Europe, Anti Jo antibodies were seen in 22% of polymyositis patients (Ann Rheum Dis 2015;74:1551)
  • Other antibodies are anti PL7, anti PL12, anti signal recognitions particle (SRP), anti Mi2
  • Genetics may play a role, since more likely in certain HLAs: HLA A*01, Cw*07, DQA1*0501, DRB1*0201, C4A*Q0, DRB1, DQA1*0501, DQA1*0401, B7, DRw6, and DRB1*0803
Clinical features
  • Most common reported feature is muscle weakness (97%), typically proximal symmetric weakness that progresses slowly
  • Myalgia, dysphagia and dyspnea can be seen
  • Raynaud's is also be seen but is more common in dermatomyositis
  • Clinicopathologic diagnosis
  • Histology shows myopathic features with inflammation; however, review of drugs and clinical history is required
  • Serum CK (creatine kinase) and aldolase are elevated in 90%
  • Elevated sedimentation rate (ESR) of 50 - 100 in 40% and an ESR above 100 in 10% (Ann Rheum Dis 1993;52:857)
Radiology description
  • MRI T2 may show edema, which is useful in muscle biopsies
Prognostic factors
  • Response to therapy is variable - older patients at diagnosis do worse
  • In one study, the 10 year survival rate was 50%; the main cause of death was circulatory followed by "musculoskeletal" (Clin Rheumatol 2006;25:234)
  • Dysphagia and recalcitrance to therapy are poor prognostic factors
Case reports
Gross description
  • Skeletal muscle gross findings are non specific
Microscopic (histologic) description
  • Skeletal muscle shows myopathic features - occasional small rounded myofibers with mildly increased internal nuclei
  • Individual myofibers show degeneration with possible macrophage infiltration to clear the necrotic myofiber
  • May be myofiber regeneration with basophilic myofibers with large nuclei and prominent nucleoli
  • Should be a prominent endomysial inflammatory infiltrate (this can be attenuated if the biopsy is after steroids/therapy)
  • Lymphocytes invading non necrotic myofibers are a diagnostic finding
  • Occasional cases show only lymphocytes associated with necrotic myofibers and the possibility of polymyositis can be suggested if clinically applicable
  • Biopsy can also show mild to moderate fibrosis
Microscopic (histologic) images

Contributed by Meggen Walsh, D.O., M.S., P.A.

Various images

Cytology description
  • Of little to no benefit, could see non specific lymphocytes
Positive stains
  • HLA class I is upregulated, with increased staining of myofibers
  • CD3 - CD8+ positive T lymphocytes may be prominent
  • Presence of non necrotic myofibers being invaded by T lymphocytes may be diagnostic
Negative stains
Differential diagnosis
  • Immune Mediated Necrotizing Myopathy: can occur post-statins or de novo; myopathic features, necrosis and myofiber regeneration, but no inflammatory infiltrate
  • Inclusion body myositis:
    • Similar inflammatory pattern
    • Rimmed vacuoles may be difficult to find or not be "classic"
    • Presence of occasional granular staining suggestive for vacuoles using LC3 or ubiquitin requires clinical correlation to determine whether these features fit best with PM or IBM;
    • Some cases are seen with positive anti-SRP (signal recognition particles) (Ann Rheum Dis 2006;65:1635)
  • Statin therapy and post-statin therapy: can appear similar to the findings of PM: thus clinical and laboratory correlation may be required
Back to top
Image 01 Image 02